In such an electrically operated braking system, there has been a need or desire to increase the wheel braking force for a given drive force or torque generated by an electric motor used for the disc brake. JP-U-5-22234 proposes a conventional braking system, which is arranged in an attempt to meet the above-indicated need. In this conventional braking system, a boosting mechanism is provided between an electric motor and friction pads of a disc brake, so that the drive force generated by the motor is boosted by the boosting mechanism before it is transmitted to the friction pads. For producing a relatively large wheel braking force, however, the motor and the boosting mechanism in this conventional braking system are subject to a comparatively large load, and therefore tend to have relatively large sizes, leading to an accordingly increased size of the motor-driven disc brake.